1. Field of the Invention
This invention relates to a cathode clamping circuit apparatus for clamping cathode beam currents of color signals with differences in intensities among the beam currents compensated.
2. Description of the Prior Art
A cathode clamping circuit apparatus is known which produces dc voltage components for color signals supplied to a cathode ray tube (CRT) and clamps the color signals with the dc voltages respectively. A voltage of the dc voltage component for each color component signal is obtained from the sum of a bright voltage and sub-bright voltage. The bright voltage is commonly used for all color component signals. While the sub-bright voltages are individually trimmed for respective color component signals. The bright voltage is manually changed by a bright control variable resistor. The sub-bright voltages are changed by sub-bright control variable resistors respectively. That is, the cathode clamping apparatus comprises a variable voltage source for producing a bright voltage and a set of cathode clamping circuit for clamping respective color signals with sum voltages of the bright and sub-bright voltages respectively. The set of cathode clamping circuits for color signals, for example, red (R), green (G), and blue (B), are trimmed to determine a black level of a color CRT. That is, the cathode clamping circuit apparatus compensates difference among CRT beam current of R, G, and B to reproduce the black level suitably.
FIG. 2 is a schematic circuit diagram of such a cathode clamping circuit apparatus 55 of a prior art. The cathode clamping circuit apparatus 55 produces dc voltage components of color signals applied to a color CRT. The voltage value of each of dc voltages is obtained from a sum of the bright voltage common to respective color signals and the sub-bright voltage for compensating differences in intensity among beam currents of R, G, and B as mentioned above. It is necessary that the bright voltage should be changed commonly among R, G, and B and the sub-bright voltages are changed individually. Therefore, each of the dc voltage components is changed with a variable resistor 21, connected between a cathode of the shunt regulator and a reference input thereof. The sums of bright voltage and the sub-bright voltages are trimmed to determine a black level on the CRT. Therefore, at least two cathode clamping circuits are necessary to adjust the black level. A transistor 6 clamps a color component signal with the sum voltage of a bright voltage and a sub-bright voltage when the clamp pulse generation circuit 8 detects a clamp level, pedestal level, or black level. The variable voltage source 13 generates the bright voltage. A shunt regulator 10 generates one of the sub-bright voltages.
Hereinbelow will be described a structure of the shunt regulator 10. FIG. 3 is a block diagram of the shunt regulator shown in FIG. 3 and is common to an embodiment of the invention. In FIG. 3, the shunt regulator 10 comprises a current source 33, an error amplifier 34, an output transistor 32, and a reference voltage source 35. It has a cathode terminal 38, a reference terminal 39, and an anode terminal 40. Operation of the shunt regulator 10 is as follows:
An internal reference voltage source 35 is connected to an inverting input of the error amplifier 34. A reference input terminal 39 is connected to a non-inverting input terminal of the error amplifier 34. The error amplifier 34 so produces an output as to maintain a balance between inverting and non-inverting input. The output of error amplifier 34 is applied to the output transistor 32 to control the output transistor 32.
In this state, a voltage value V.sub.KA, that is, a sub-bright voltage value of the cathode terminal 38 of the shunt regulator 10 is given by: EQU V.sub.KA =V.sub.REF (1+R.sub.21 /R.sub.22)+I.sub.REF .multidot.R.sub.21
when the anode terminal 40 of the shunt regulator 10 is grounded and a non-inverting input current I.sub.REF &lt;&lt;1. The reference R.sub.21 shows a resistance of the resistor 22 and the reference R.sub.22 shows a resistance of the resistor 21.
Here, the term I.sub.REF .multidot.R.sub.21 can be neglected if the resistance R.sub.21 is assumed tens kilo-ohms because in the actual circuit, the order of I.sub.REF is assumed several microamperes. This is because the error amplifier 34 is used as a non-inverted type. Therefore, the voltage value V.sub.KA is given by: EQU V.sub.KA =V.sub.REF (1+R.sub.21 /R.sub.22)
The voltage value V.sub.KA can be changed in accordance with the resistance of the variable resistor 21.
In the cathode clamping circuit apparatus 55 using the variable resistor 21 as mentioned in the prior art, there are problems, namely a decrease in reliability due to poor contact at the variable resistor 21. This is due to the fact that the variable resistor 21 has a mechanical structure and it is difficult to realize automation of trimming the variable resistor 21 at a factory. The presence of the variable resistor 21 is an obstacle factor in trimming at a factory.